Brushless DC motor

Introduction

A brushless DC motor is electric engine where the stator is a classic 3 phase stator and the rotor has surface mounted permanent magnets. It is equivalent to a reversed DC commutator motor, in which the magnet rotates while the conductors remain stationary. BLDC motor has no commutator and brushes, it requires electronic control for operation. They often have external position sensors used by control unit, but sensorless control is also possible.

Where to find?

Very popular source of small BLDC motors are old CD-ROMs, floppy drives and streamers. To get more power from these small engines many people replace original magnets with strong neodyme magnets and rewind the winding. You should look for motors where housing and magnets rotate (they are easer to dismantle).

BLDC in underwater environment

In theory, you could put BLDC motor into water and it should work (assuming you isolate wire connections). Wires and stator are isolated. But in case of a mechanical damage, you can easily destroy isolation of wires (it is in fact an isolation paint). A better isolation is needed. The easiest solution is to put stator into epoxy. Compared to classic DC motor you only need to isolate stator! No o-rings nor magnetic couplers!

Summary

Advantages

  • Magnetic coupler like isolation
  • No gearbox, speed controlled by electronic unit
  • Good size/power ratio
  • Water cooling

Disadvantages

  • Modifying CD-ROM/floppy motors can be tricky
  • Need dedicated BLDC controller

My motor

My plan was to build complete engine for underwater vehicle based on modified CD-ROM motors. Unfortunately because of lack of free I never realized this project and do not plan to finish ti in the near future. In addition it is easier to build thruster using classical motor and magnetic coupler to achieve the same results.
Incomplete motor with the propeller.

Motor

  1. Replace original magnets with strong neodyme magnets
  2. Rewind
  3. Isolate stator
  4. new rotor and bearings (optional)

Controller

After examing BLDC projects available on the net I decided to build my own controller. See BLDC controller page for more details.

Step by step instruction

I did not build any working engine, so this description is not yet complete.

Preparation

Below is the list of items you need to modify CD-ROM motor:
  • Old CD-ROM/DVD drives, cost 0€
  • Small neodyme magnets, cost 1€ for 40
  • Two types of glue:
    • "Superglue" like, to quickly and temporary attach magnets to rotor, cost about 1€
    • Epoxy glue, to permanently attach magnets to rotor, cost about 2.5€
  • Optional lathe, to make own rotor or new bearings
  • Hammer, vice, drill, screwdriver, etc.

Removing motor from CD-ROM

That is easy part. Use screwdriver and remove motor that spins the disk. It is always located exactly in the center of the box. All parts can be disassembled using only screwdriver, you do not need to use hammer or things like that.

Disassembling motor

This is the moment you can use the hammer and vice. Motor has no serviceable parts, so brutal force is the only method that can be used. The best method is to use vice and additional tools to slowly separate parts. If you use hammer, use it "softly" in a way you do not damage bearings or stator. It is very important that you do not damage stator, it is the only part that can not be replaced.

Replacing magnets

First remove old magnet from rotor. Original magnet is gum like substance, so it is easy to remove it with screwdriver. In place of original magnet, place a number of strong neodyme magnets. You must select size of magnets to fit properly empty place. Air gap should be minimal. I use 5x5x1mm magnets. Number of used magnets must be chosen carefully, see link sections to read more.

New winding

Direct link to Excel winding calculator.

Links

Unfortunately most of the brushless pages is in German language. But pictures and diagrams are still readable.

RC Controllers

Amateur BLDC